2 The use of the gravimetric GEOid of TUNisia: GEOTUNV. Corchete*1, C. Jallouli2, M. Chourak3,4 and N. Rebai21Higher Polytechnic School, University of Almeria, ALMERIA, Spain2Faculté des Sciences de Tunis, University Tunis El Manar, TUNIS, Tunisia3Faculté Polidisciplinaire d'Errachidia, University of Moulay Ismaïl, B.P. 509 BOUTALAMINE, Morocco4NASG (North Africa Seismological Group)AECIThe relation between orthometric height (H) and ellipsoidal (h) is the undulation of the geoid (N):H = h - NSurface of the EarthThe use of the GPS for levelling requires a geoid model to get orthometric heights from ellipsoidal heights.GEOid model for TUNisia: GEOTUNASurface of the EarthBHGeoidChNANBEllipsoidHA = hA - NADHBA = DhBA - DNBAHB = hB - NBDNBA = NB – NA 0DHBA DhBAAcknowledgements. The authors are grateful to the National Geophysical Data Center (NGDC), the Bureau Gravimetrique International (BGI) and the United States Geological Survey (USGS); who have provided the data used in this study. NGDC and USGS have supplied the elevation data required to compute the necessary terrain corrections, through the databases: ETOPO2 and SRTM 90M (available by FTP internet protocol). The Agency of International Cooperation (AECI, Spain) supported this research partially, through the project A/5830/06.*Corresponding author: Fax ;

3 The first high-resolution gravimetric GEOidfor TUNisia: GEOTUNV. Corchete*1, C. Jallouli2, M. Chourak3,4 and N. Rebai21Higher Polytechnic School, University of Almeria, ALMERIA, Spain2Faculté des Sciences de Tunis, University Tunis El Manar, TUNIS, Tunisia3Faculté Polidisciplinaire d'Errachidia, University of Moulay Ismaïl, B.P. 509 BOUTALAMINE, Morocco4NASG (North Africa Seismological Group)AECIComputation of the geoidWe have followed the computation method detailed by Corchete et al. (2005) for the calculation of the gravimetric geoid shown in Figure 6. Following this method, a complete data set consisting of: free-air gravity anomalies, a geopotential model and a high precision DTM; is necessary.The gravity data used have been 8628 points, distributed over the study area as it is shown in the Figure 1.The EIGEN-GL04C model (Förste et al., 2006) represents a major advance in the modelling of the Earth’s gravity and geoid (Figure 2). Therefore, this geopotential model has been considered in this study.Any gravimetric geoid computation must use anomalies that have been reduced to the geoid. This involves the computation of the terrain correction (Figure 4) and the indirect effect (Figure 5) on the geoid, which are computed from a DTM (Figure 3). This DTM is based on the SRTM (Shuttler Radar Topography Mission) and ETOPO2.Figure 1. Gravity data.Figure 2. EIGEN-GL04C geoid.Figure 4. Terrain correction.Figure 5. Indirect effect.Acknowledgements. The authors are grateful to the National Geophysical Data Center (NGDC), the Bureau Gravimetrique International (BGI) and the United States Geological Survey (USGS); who have provided the data used in this study. NGDC and USGS have supplied the elevation data required to compute the necessary terrain corrections, through the databases: ETOPO2 and SRTM 90M (available by FTP internet protocol). The Agency of International Cooperation (AECI, Spain) supported this research partially, through the project A/5830/06.Figure 6. Gravimetric GEOid of TUNisia (GEOTUN).REFERENCESCorchete, V., M. Chourak and D. Khattach. The high-resolution gravimetric geoid of Iberia: IGG Geophys. J.Int., 162, 676–684, 2005.Förste, C., F. Flechtner, R. Schmidt, R. König, U. Meyer, R. Stubenvoll, M. Rothacher, F. Barthelmes, H. Neumayer,R. Biancale, S. Bruinsma, J.-M. Lemoine and S. Loyer. A mean global gravity field model from thecombination of satellite mission and altimetry/gravimetry surface data: EIGEN-GL04C. GeophysicalResearch Abstracts, Vol. 8, 03462, 2006.Figure 3. Digital Terrain Model (DTM).*Corresponding author: Fax

4 Bouguer anomaly map of the Moroccan areaV. Corchete*1, M. Chourak2,3 and D. Khattach41Higher Polytechnic School, University of Almeria, ALMERIA, Spain2Faculté Polidisciplinaire d'Errachidia, University of Moulay Ismaïl, B.P. 509 BOUTALAMINE, Morocco3NASG (North Africa Seismological Group)4Faculté des Sciences, University of Mohamed I, OUJDA, MoroccoAECIComputation of the Bouguer anomaliesSince the gravity data set consisting of point data anomalies distributed randomly (Figure 1), we need to interpolate these data to obtain a regular data grid (Corchete et al. 2007). Before the interpolation, the short-wave effects that appear in the gravity anomaly field, associated to the short-wave topography and bathymetry, must be corrected (Corchete et al., 2005). Thus, we can compute a regular grid of free-air gravity data, as it is shown in Figure 3.When we have obtained the free-air gravity anomalies gridded, we can compute the Bouguer complete anomalies, as it shown in Figure 5, considering the terrain correction (Figure 4) previously obtained from a DTM of the study area (Figure 2). This DTM is based on the SRTM (Shuttler Radar Topography Mission) and ETOPO2.Figure 1. Gravity data.Figure 2. Digital terrain model (DTM).Figure 3. Free-air anomaly map.Figure 4. Terrain correction.Acknowledgements. The authors are grateful to the National Geophysical Data Center (NGDC), the Bureau Gravimetrique International (BGI) and the United States Geological Survey (USGS); who have provided the data used in this study. NGDC and USGS have supplied the elevation data required to compute the necessary terrain corrections, through the databases: ETOPO2 and SRTM 90M (available by FTP internet protocol). The Agency of International Cooperation (AECI, Spain) supported this research partially, through the project 4/04/P/E.Figure 5. Bouguer anomaly map of the Moroccan area.REFERENCESCorchete, V., M. Chourak and D. Khattach. The high-resolution gravimetric geoid of Iberia: IGG Geophys. J. Int., 162, 676–684, 2005.Corchete, V., M. Chourak, D. Khattach and E. H. Benaim. The high-resolution gravimetric geoid of Morocco: MORGEO. Journal of African Earth Sciences, 48, ,2007.*Corresponding author: Fax ;